At a minimum everyone should carry some basic form of a first aid kit with them at all times especially going out on the trails. Accidents happen and you never know what and when things will go bad so being prepared is a proactive way to ensure you're prepared for some basic first aid support.
Ideally speaking having two separate kits would be best. The logic behind this is so that if you have a minor injury there is no need to cross contaminate or even fumble through the other items for the simple patch up. Likewise when you are dealing with a trauma you don't want to be fumbling through your kit locating the necessary items. The smaller pack makes it easier to pack in your bag if you decide to leave the vehicle for a short hike too!
With this in mind here's a short list of various first aid kits that I've been able to come across. I have not tested many of them but I have read from various forums and members in the community/industry that recommends many of the vendors below.
Adventure Medical
Adwater Carey
Emergency Medical Products
Galls
Lifeline
Mountain Equipment Company
Zee Medical
These are just sample links that I've been able to compile so please post up other places that you've visited to purchase your first aid needs. Also keep in mind that the bigger ticket items are not as complete for trauma related injuries as they are really "basic" first aid kits with more stuff. For a full featured kit you'll want to ultimately build your own from scratch. These kits are a good starting point to get what you need.
Got pets? Don't forget to have a dedicated kit for them too!